1. Field of the Invention
This invention relates generally to radio frequency (RF) receivers and, more particularly, to RF mixers.
2. Related Art
To optimally process radio frequency (RF) signals, most RF receivers convert received RF signals to lower frequencies termed baseband frequencies. The filtering and amplification performed in processing the RF signal at baseband requires less expensive electrical components than those required for accurate processing at RF. Further, these signal processing components perform best at baseband and thus are able to improve RF receiver gain, dynamic range and stability.
Typically, RF receivers employ mixers to convert the received RF signal to a lower frequency while conserving the modulation information contained in the received signal. Frequency shifting occurs by mixing, or taking the difference between, the received RF signal and a reference frequency from a local oscillator (LO). The difference between the RF signal and the LO frequency is the lower frequency or baseband frequency.
The process of converting the RF signal to a lower frequency is called downconverting. The RF receiver functions to downconvert a received RF signal to baseband. Direct conversion or homodyne receivers directly downconvert the received RF signal to baseband by mixing the received RF signal with a LO frequency equal to the received RF frequency.
An example of a prior art mixer, commonly known as a xe2x80x9cGilbert cell,xe2x80x9d is shown in FIG. 1. The mixer includes transistors Q0-Q3 and a RF input section using transistors Q4, Q5. Input to the Gilbert cell is a LO voltage where the oscillation of the LO voltage causes a current to commutate between the pair Q0, Q2 and the pair Q1, Q3. This commutation action produces a mixing of the RF signal with the local oscillator signal to produce an output baseband signal that is a downconverted received RF signal. Further, a Gilbert cell often employs a diode predistortion circuit to linearize the upper quad of transistors, as shown in FIG. 2. The diode predistortion circuit includes transistors Q6-Q9.
Since a direct conversion receiver, as shown in FIG. 3, directly mixes (often using Gilbert cells as shown in FIGS. 1 and 2) the received RF signal with the LO signal, the LO signal leaks through the RF signal input port of the mixer, as shown in FIG. 3, and may be reflected at the antenna. The reflected LO signal is indistinguishable from the received RF signal, and so is downconverted at the mixer by the LO signal present on the mixer""s LO port and xe2x80x9cself-mixedxe2x80x9d. Self mixing results in a dc offset, which distorts the processing of the received RF signal. Thus, the sensitivity of the RF receiver may be limited by the reflected LO signal. Further, the ability to shift a signal in frequency with minimal added distortion may be important because the RF receiver may be frequency dependent. RF receivers must properly downconvert to effectively perform.
One approach to the problem of self-mixing can be to use a heterodyne receiver. As shown in FIG. 4, a heterodyne receiver performs downconversion in two stages using two mixers. A first mixer 402 converts the received RF frequency to an intermediate frequency (IF), which is not equal to the received RF frequency. The IF signal is amplified in a low noise amplifier 403 and then a second mixer 404 downconverts the IF to baseband. Since the downconversion may be performed in two stages, heterodyne receivers do not suffer from the xe2x80x9cself-mixingxe2x80x9d problem. However, using a heterodyne receiver implementation requires using more discrete components, which translates into higher-cost RF receivers.
Another approach to the problem of self-mixing may be to use a low-IF receiver architecture. A low-IF receiver performs downconversion by using a single mixer stage to frequency translate the received signal to a low intermediate frequency which is typically on the order of one or two channel bandwidths. Then, the signal may be passed through an analog-to-digital converter (ADC) where a digital multiplication may be performed. Finally, image rejection filtering may be performed to remove imageband attenuation. Since the downconversion may be performed to a low-IF, low-IF receivers do not suffer from the xe2x80x9cself-mixingxe2x80x9d problem. However, using a low-IF implementation requires image rejection components that add to the cost and expense of RF receiver design.
Since a direct conversion receiver can be a preferred approach to RF receivers in some applications, an improved direct conversion receiver which downconverts without an unwanted dc offset is needed.
Under one embodiment of the invention, a subharmonic mixer that includes a mixer core of two switching stages and a RF input section is presented. The mixer core has a LO interface for receiving a local oscillator signal, a BB output for providing a BB output signal, and first and second inputs for connecting to an RF input section. The RF input section includes a current mode signal and a RF input for receiving a RF input signal. The RF input section includes a first transistor having a first terminal coupled to the first input of the mixer core to supply a first current and the first current responsive to the RF input signal, a second transistor having a second terminal coupled to the second input of the mixer core to supply a second current and the second current responsive to the RF input signal. As will be shown, the RF input section may take many forms.
The mixer core of two switching stages includes a first switching stage of four transistors connected as a doubly-balanced mixer coupled in series with a second switching stage of four transistors connected as a doubly balanced mixer. The first switching stage includes two input terminals, each connected to the base of a respective transistor in a differential pair, to which a LO voltage is applied and two output terminals, each connected to the input of the second switching stage. The second switching stage also includes two input terminals, each connected to the base of a respective transistor in a differential pair, to which a LO voltage is applied.
Under another embodiment of the invention a circuit is provided that improves mixer gain by utilizing two switching stages in a direct downconversion receiver. The first switching stage mixes a received RF signal to an intermediate frequency that may be approximately one-half the received RF signal frequency. The second switching stage mixes the intermediate frequency to baseband. By connecting the two switching stages in series, current may be reused and harmonic content from the first stage may be fed into the second stage thus improving the mixer gain.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.